Phosphorus mass load and outflow concentration relationships in stormwater treatment areas for Everglades restoration

Mass loading and outflow phosphorus (P) relationships were investigated for four stormwater treatment area (STA) wetlands in south Florida. These systems, ranging in size from 350 to 2670 ha, were constructed by the South Florida Water Management District (SFWMD) for Everglades restoration, and approaches currently are being investigated for optimizing their design and management. We analyzed 2–7 years of P removal data from 10 independent STA process trains using system classifications based on dominant vegetation type, which was either emergent aquatic vegetation (EAV) or submerged aquatic vegetation (SAV), and prior land use, which was either recently farmed (RF) or historic wetland (HW). We found that a 1–2 year history of mass loading rates (MLR) at or below ∼1.3 g P/m2/year in STA process trains provided a high likelihood of achieving outflow total P (TP) concentrations less than ∼30 μg/L. Statistical analyses revealed that P removal performance of SAV and EAV-HW systems was generally superior to that of EAV-RF systems. These performance differences were corroborated with data from seven other non-STA Florida-based treatment wetlands. Furthermore, in the subset of SAV and continuously flooded EAV-HW data with P MLRs at or below ∼2 g/m2/year, outflow P concentrations were consistently between 10 and 20 μg/L, mass removal efficiencies were consistently above 85%, and the wetlands demonstrated a substantial resilience to small-to-moderate pulsed inflow P loads. Despite 16 occurrences in these full-scale STA data of annual flow-weighted mean outflow P concentrations between 10 and 20 μg/L, no significant MLR–P relationships were identified for targeting specific P concentrations in this range.